## Functions Modeling Change: A Preparation for Calculus, 5th Edition

$$2\sqrt{23}$$
Plugging in $x=4$: $$\sqrt{20+2\cdot \:4^2} \\ 2\sqrt{13}$$ Plugging in $x=6$: $$\sqrt{20+2\cdot \:6^2} \\ 2\sqrt{23}$$